Field of the Invention
The present invention relates to an optical modulator for recording information on an optical disk in an optical pickup for the reproduction of recorded information, and more particularly, to a fast responding optical modulator for high-density recording.
Generally, a light source of a short wavelength is essential for the high-density recording of optical information. For the light source generating a short wavelength light, a second harmonic generation (SHG) laser is currently favored in the high-density recording area using a wavelength of 532 nm or below. However, unlike a conventional semiconductor laser, since SHG laser cannot provide a modulated light itself, an external modulator for modulating light is required during the recording operation. Accordingly, for modulating light from the SHG laser, an optical modulator such as an electro-optical modulator or acousto-optical modulator is usually employed.
A magnetic domain on an optical disk can be adjusted easily at fast rise and fall times of a modulated light. Thus, when external optical modulation is employed, the improvement of a fast response is required.
The rise time of the optical modulator is related to the optical modulation efficiency. As shown in the graph of FIG. 1, the rise time becomes faster and an optical efficiency becomes lower, as the spot size of a light beam which passes through the center of the optical modulator becomes smaller. Therefore, there is a need for optimizing the optical efficiency and the rise time in configuring an optical pickup using an optical modulator. For example, when the quantity of an optical spot is determined within the hatched area in the graph of FIG. 1, the result satisfies the rise time of 8 ns and below and the optical efficiency of 80% and over.
Meanwhile, because such conditions as light spreading angle of laser or beam diameter are not exactly congruous with each other even when a laser of the same specification is adopted, there is a non-uniformity between the optical efficiency and rise time of each optical modulator, which must be taken into consideration.
FIG. 2 shows a conventional optical modulator. A light beam emitted from light source 1 travels through a collimator lens 2 and a focus lens 3 in this order and passes through the center of a optical modulator 4. To adjust an optical spot size penetrating the center of the optical modulator 4, there is a method of altering the distance (L.sub.1 +D) between light source 1 and focus lens 3, or focal length L.sub.2 of focus lens 3. By the way, in using the above method to alter the distance (L.sub.1 +D) or the length L.sub.2 of an optical system, the whole optical system of an optical pickup must be reset or a focus lens 3 has to be remanufactured, since the total distance L.sub.T (L.sub.1 =D+L.sub.2) between light source 1 and optical modulator 4 is altered.
On the other hand, when focal lens 3 is defocused without altering the distance L.sub.T between light source 1 and optical modulator 4, the optical spot size is enlarged rather than reduced in size, so that a faster rise time becomes impossible. This is due to the fact that distance D between collimator lens 2 and focus lens 3 is greater than the sum of the focal lengths of the lenses 2 and 3.